Typically, in computed tomography (CT) imaging systems, a rotatable gantry includes an x-ray tube, detector, data acquisition system (DAS), and other components that rotate about a patient that is positioned at the approximate rotational center of the gantry. X-rays emit from the x-ray tube, are attenuated by the patient, and are received at the detector. The detector typically includes a photodiode-scintillator array of pixelated elements that convert the attenuated x-rays into photons within the scintillator, and then to electrical signals within the photodiode. The electrical signals are digitized and then received within the DAS, processed, and the processed signals are transmitted via a slipring (from the rotational side to the stationary side) to a computer or data processor for image reconstruction, where an image is formed.
The gantry typically includes a pre-patient collimator that defines or shapes the x-ray beam emitted from the x-ray tube. X-rays passing through the patient can cause x-ray scatter to occur, which can cause image artifacts. Thus, x-ray detectors typically include an anti-scatter grid (ASG) for collimating x-rays received at the detector.
Imaging data may be obtained using x-rays that are generated at a single polychromatic energy. However, some systems may obtain multi-energy images that provide additional information for generating images.
Dose management in CT has become increasingly important in recent years. Thus, in a typical CT scanner, a pre-patient collimator is used to limit x-ray exposure only to the region of interest (ROI) for imaging. To achieve this, collimator apertures made typically of tungsten are included that provided for a different beam width. In general, the pre-patient collimator is used to reduce overbeaming and to control it. Overbeaming is commonly referred to as an amount of the x-ray beam that is incident to the patient which lies outside the active detector area in the Z-axis. Because the focal spot in the Z-axis is not a point, there will typically be overbeaming due to the penumbra from the focal spot. The penumbra refers to the partial outer region that falls outside the umbra, and the umbra typically refers to a full inner region of the x-rays that pass through the patient from the source focal spot.
The penumbra is fixed for x-ray apertures in Z. However, the ratio of overbeaming to the x-ray aperture will decrease with the size of the aperture. Consequently, the dose to the patient will increase when the total beam width decreases. Some manufacturers design the aperture(s) having moving edges or “Z-axis focal spot tracking” to track focal spot umbra and penumbra.
In a design having moving edges, an algorithm tracks focal spot motion and controls position of the aperture through which the x-rays pass. This typically results in complex and expensive hardware to account for the geometric layout of the detector plane with respect to the focal spot, and the distances therebetween. Such a design can improve dose efficiency by maintaining a small aperture without affecting image quality. In such a design, when the focal spot moves in the Z-axis (such as due to mechanical or thermal drift), the aperture is adjusted and aligned to cover only the beam for the desired ROI. Such a design includes sophisticated control of the slit with high precision motors, typically including two or more motors.
Thus, there is a need to improve tracking of the focal spot.